1. Field of the Invention
This invention relates to a direct-injection spark-ignition internal combustion engine, and specifically, a technique of carrying out stratified charge combustion using spray guide method and wall guide method.
2. Description of the Related Art
In a direct-injection spark-ignition internal combustion engine in which fuel is injected directly into a combustion chamber, a technique called spray guide method is known, in which an injector is fitted to an upper wall of the combustion chamber, in a central area thereof, and an ignition plug is arranged with its electrode part positioned within or near a fuel injection area, so that stratified charge combustion can be carried out by directly igniting fuel spray formed from fuel injected by the injector and vaporized in part.
In the stratified charge lean combustion by this spray guide method, particularly in low-load operation, fuel injection quantity is small and therefore injection period is short, so that the period of time in which ignition can be carried out is short. Thus, there is a problem that in low-load operation, stable combustion region is narrow and stability of combustion is not sufficient.
In order to carry out stratified charge combustion stably in the direct-injection spark-ignition internal combustion engine, a technique has been developed, in which intake air flowing in through an intake port is caused to rise as tumbling flow by a cavity formed in the top face of a piston, and fuel is injected toward the side to which the tumbling flow rises (Japanese Unexamined Patent Publication No. Hei 11-210472).
However, in wall guide method including the type using the tumbling flow, conditions for combustion (fuel injection timing, ignition timing, etc.) vary greatly depending on engine speed and load on the internal combustion engine. Hence, if the engine speed or load changes suddenly and unexpectedly, misfire can happen due to unmatched fuel injection timing, ignition timing, etc. Hence, conventionally, in order to maintain stable combustion, while the internal combustion engine is in transitional operation, ordinary uniform charge combustion is carried out.
As stated above, although the stratified charge combustion has advantages such as improving fuel economy, the stratified charge combustion is carried out limited to specified operational status since it tends to become unstable depending on operational status of the internal combustion engine.
Further, in the spray guide method, fuel spray gradually moves away from the ignition plug, and flames generated by ignition by the ignition plug spread within the combustion chamber following the fuel spray. Thus, combustion progresses relatively slowly. Hence, exhaust gas temperature is relatively low, which causes lowering of the efficiency of conversion by a catalytic converter provided in an exhaust path.